The invention relates to the field of magnetic actuators, and in particular to magnetic tripping devices which are designed to respond to the magnitude of a voltage or current by operating if this magnitude exceeds a certain value. Rather than being designed to provide maximum force for a given size or electrical power input, actuators of this type have details of magnetic and physical construction selected to maximize the accuracy of tripping level--that is, there should be only a small variation between the current or voltage which may cause tripping, and the highest value for which tripping may not occur.
An especially important use of magnetic tripping devices is to provide fast tripping of an electrical circuit breaker in response to a relatively large overcurrent condition, without tripping due to a brief overcurrent such as a motor starting current. In these circumstances the permissible inrush current may be three or more times the long-term overcurrent limit; but it is desirable that tripping occur rapidly in the event of a larger overload, to avoid damage to equipment obtaining current from the breaker.
One well-known method of adjusting the actuating level (current or voltage) of a relay or solenoid is to adjust the spring forceholding the armature against a stop. To do this, either the remote end of the spring must be held by an adjustment device, or springs must be selected or trimmed. The former method is widely used for relays having a clapper which is attracted toward an electromagnet subassembly, and is usable with solenoids having an exterman spring. However, when it is desirable to minimize size or moving mass, particularly of a solenoid, an internal spring is apt to be the designer's choice. Selecting or trimming springs then requires repeated disassembly and reassembly.
In multi-pole circuit applications, the designer faces a compromise: a solenoid used to release a trip latch will make it easier to trip all poles quickly, and because only a small force is required to actuate the latch, a fast solenoid is easier to achieve in a compact construction. However, if very heavy spring forces are to be avoided, a hammer action is desirable, in which the solenoid directly pulls open a movable contact arm. Such a solenoid is designed to develop very high force when a short circuit current flows through the coil. Making such a solenoid also accurately and economically adjustable has not been possible to date.